The present invention relates generally to electrical switching devices. More particularly, the present invention relates to electrical switching devices with an improved joint system joining an insulating bushing to a circuit interrupter tank.
A high voltage circuit breaker is a device used in the transmission and distribution of three phase electrical energy. When a sensor or protective relay detects a fault or other system disturbance on the protected circuit, the circuit breaker operates to physically separate current-carrying contacts in each of the three phases by opening the circuit to prevent the continued flow of current. In addition to its primary function of fault current interruption, a circuit breaker is capable of load current switching. A circuit switcher and load break switch are other types of switching device. As used herein, the expression xe2x80x9cswitching devicexe2x80x9d encompasses circuit breakers, circuit switches, dead tank breakers, live tank breakers, load break switches, reclosers, and any other type of electrical switch.
The major components of a circuit breaker or recloser include the interrupters, which function to open and close one or more sets of current carrying contacts housed therein; the operating mechanism, which provides the energy necessary to open or close the contacts; the arcing control mechanism and interrupting media, which interrupt current and create an open condition in the protected circuit; one or more tanks for housing the interrupters; and the bushings, which carry the high voltage electrical energy from the protected circuit into and out of the tank(s) (in a dead tank breaker). In addition, a mechanical linkage connects the interrupters and the operating mechanism.
Circuit breakers can differ in the overall configuration of these components. However, the operation of most circuit breakers is substantially the same. For example, a circuit breaker may include a single tank assembly which houses all of the interrupters. U.S. Pat. No. 4,442,329, Apr. 10, 1984, xe2x80x9cDead Tank Housing for High Voltage Circuit Breaker Employing Puffer Interrupters,xe2x80x9d discloses an example of the single tank configuration and is incorporated herein in its entirety by reference. Alternatively, a separate tank for each interrupter may be provided in a multiple tank configuration. An example of a prior art, multiple tank circuit breaker is depicted in FIGS. 1, 2, 3, and 4. Circuit breakers of this type can accommodate 72 kV, 145 kV, 242 kV, and 362 kV power sources.
The circuit breaker shown in FIG. 1 is commonly referred to as a xe2x80x9cdead tankxe2x80x9d because it is at ground potential. FIG. 1 provides a front view of a three phase or three-pole circuit breaker having three entrance insulating bushings, 10, 11, and 12, that correspond to each respective phase. The insulating bushings may be comprised of porcelain, composite, or a hardened synthetic rubber sufficient to withstand seismic stresses as well as stresses due to the opening and closing of the interrupter contacts within the device. In high voltage circuit breakers, the bushings for each phase are often mounted so that their ends have a greater spacing than their bases to avoid breakdown between the exposed conductive ends of the bushings.
The circuit breaker is comprised of three horizontal puffer interrupter assemblies enclosed in cylindrical tanks 15, 16, and 17. Current transformers assemblies and 21 (referring to FIG. 2), which comprise one of more current transformers and their exterior housing, are located underneath the insulating bushings on the exterior of the breaker to facilitate their replacement in field. Current transformers 20 and 21 measure outgoing current.
FIG. 2 provides a side view of the three-pole circuit breaker of FIG. 1 that shows the corresponding exit insulating bushing, 13, of the interrupter assembly housed in tank 15. FIG. 2 illustrates how entrance insulating bushing 10 and exit insulating bushing 13 is associated with tank 15. The entrance and exit insulating bushings for the interrupters in tanks 16 and 17 (not shown in FIG. 2) are arranged in a similar fashion. The devices, illustrated in FIGS. 1 through 3, have 3 pairs of entrance and exit bushing insulators, or a total of 6 bushing insulators.
Referring to FIG. 1 and FIG. 2, the three interrupter tank assemblies are mounted on a common support frame 19. The operating mechanism that provides the necessary operating forces for opening and closing the interrupter contacts is contained within an operating mechanism housing or cabinet 18. The operating mechanism is typically mechanically coupled to each of the interrupter assemblies through a common linkage such as a drive cam. The operating mechanisms can be, but are not limited to, compressible springs, solenoids, or pneumatic-based mechanisms.
FIG. 3 is a partial, cross-sectional view of the interrupter assembly housed within cylindrical tank 15 and shown in FIG. 1 and FIG. 2. A typical circuit interrupter is comprised of stationary and movable contact assemblies 31 and 23, respectively. Entrance insulating bushing 10 houses a central conductor 22 which supports movable contact assembly 23 within conductive tank 24. Movable contact assembly 23 is affixed to an insulating tube 25 through which a linearly operating rod 26 extends. Rod 26 operates movable contact 27 between its open and closed position in a well-known fashion.
Exit insulating bushing 13 houses a central conductor 30 which is connected to the stationary contact assembly 31 and is also supported within conductive tank 24. An insulating tube 32 extends between the stationary contact assembly 31 and the movable contact assembly 23.
The interior volume of tank 24, as well as the entrance and exit insulating bushings 10 and 13, are preferably filled with an inert, electrically insulating gas such as SF6. The electrically insulating gas fulfills many purposes. The arcing contacts within both the stationary and movable contact assemblies are subject to arcing or corona discharge when they are opened or closed. Such arcing can cause the contacts to erode and disintegrate over time. Current interruption must occur at a zero current point of the current waveshape. This requires the interrupter medium to change from a good conducting medium to a good insulating or non-conducting medium to prevent current flow from continuing. Therefore, a known practice (used in a xe2x80x9cpufferxe2x80x9d interrupter) is to fill a cavity of the interrupter with an inert, electrically insulating gas that quenches the arc formed. During operation of the contacts in assemblies 23 and 31, a piston, which moves with the movable contact in assembly 23, compresses the gas and forces it between the separating contacts and toward the arc, thereby cooling and extinguishing it. The gas also acts as an insulator between conductive parts within housing 15 and the wall of tank 24.
FIG. 4 provides a detailed, cross sectional view of how an insulating bushing 33, similar to entrance insulating bushing 10 and exit insulating bushing 13 in FIG. 1 through FIG. 3, is associated with tank 34 in the prior art. Insulating bushing 33 is joined to tank 34 through the use of a mounting flange, 35, and a plurality of fasteners 36. Fasteners 36 are inserted parallel to the axis of mounting flange 35 (and the axis of insulating bushing 33) and spaced equidistantly around the periphery of flange 35. The diameter of mounting flange 35 must be sufficient to allow adequate space for fasteners 36. Moreover, the material selected for flange 36 must be suitable to withstand continuous exposure to the weather.
In addition for the joint being strong, it is also important that the joint between the insulating bushing 33 and tank 34 be gas tight and moisture tight. Both insulating bushing 33 and tank 34 have hollow interiors that are preferably filled with an inert, electrically insulating gas such as SF6. Further, insulating bushing 33 contains a central conductor 37 that is connected to either a movable and stationary contact assembly (not shown in FIG. 4) within tank 34. Moisture within the interior volume of insulating bushing 33 can create an interior flashover that may prevent the bushing from operating at specified operating voltages. Lastly, insulating bushing 33 includes internal shielding 47 to reduce voltage stress at the lower end of the bushing.
As FIG. 4 further illustrates, current transformer 38, which is generally ring-shaped, is assembled over and proximate to the neck of the aperture of tank 34. Current transformers 38 are mounted external to insulating bushing 33 to allow for maximum accessibility. The external placement of current transformers 38 allow them to be replaced or changed during field use without removing bushing 33. Each bushing 33 on tank 34 typically accommodates up to three stacked current transformers 38, depending upon the accuracy and the ratio of the switching device. Current transformer 38 is contained within a weather cover, which comprises an inner can (consisting of a base 39, bottom plate 40, and back plate, 41), spacers 42, insulating gasket 43, and external cover 44. The weather cover is mounted onto the neck of tank 34 through the use of a L-shaped flange 45 that is fastened to a lip 46 (or additional flange depending upon the design of tank 40) through a plurality of fasteners 36.
Although the joinder method illustrated in FIG. 4 yields a strong joint, the use of flanges and support hardware thereby increases the size and component parts of the weather covers that house and protect the current transformers. This increased size also increases the local stresses that are imposed upon the tank due to the additional hardware required for assembly. The vertically positioned bolts can provide a path for water entry into the flange joint that may lead to corrosion. Lastly, because the current transformers are installed over the insulating bushing and the flange, the diameter of the current transformers may also be increased. Therefore, there is a need to securely join the insulating bushing and tank of an electrical switching device whereby the current transformers are protected from external elements, the number of assembly parts are reduced, and the diameter of the current transformer and its housing is minimized.
The present invention provides electrical switching devices that have a novel mounting arrangement for attaching one or more insulating bushings to a central tank without requiring the use of traditional mounting flanges. The mounting arrangement of the present invention reduces the diameter of the component parts, the number of parts required for assembly, and the stress imposed on the tank at the point of attachment. The mounting arrangement of the present invention also reduces the risk of environmental exposure of the current transformers by improving the design and profile of the weather covers. The improved design of the weather covers prevents the entry of water into the bushing-tank joint.
According to the invention, the electrical switching device has one or more insulating bushings that are laterally joined to a tank that contains the circuit interrupter assemblies. In preferred embodiments, the electrical switching device, such as the devices for 72 kV, 145 kV, 242 kV, and 362 kV three phase power, has three tanks which are supported on a common frame and six insulating bushings (or three pairs of entrance and exit insulating bushings). Other embodiments, such as the devices for 550 kV or 800 kV power, feature one large tank and two insulating bushings (or one pair of entrance and exit insulating bushings). The insulating bushings may be comprised of porcelain, silicone rubber composite, silicone composite or a hardened synthetic rubber sufficient to withstand seismic stresses as well as stresses due to the opening and closing of the interrupter contacts within the device. The bushings are preferably tubular shaped, or more preferably conical shaped, and comprise a hollow interior to house a central conductor. The bushing further comprises a central conductor that extends from the conductive tip of the bushing, through the bushing into the interior volume of the tank where the movable and stationary assemblies of the circuit interrupter assemblies are contained. The insulating bushings are further comprised of two ends: an open-ended base and a closed-end tip. A mounting ring is attached, or otherwise joined in some fashion, to a portion of the open-ended base of the insulating bushing.
The tank is preferably a cast aluminum or steel metal that can withstand high pressures and temperatures. The tank is comprised of an interior volume, to contain the circuit interrupter assemblies, and one or more apertures or openings. The open-ended base of each insulating bushing is placed over the aperture of the tank thereby forming a continuous volume. This continuous volume is preferably filled with an inert, electrically insulating gas such as SF6. The joint may further include seals or gaskets to ensure that the joint is gas tight and moisture tight.
The insulating bushing and tank are joined laterally (i.e., at an angle that is not parallel to the axis of the insulator bushing) by a plurality of fasteners. In preferred embodiments, the fasteners are positioned substantially perpendicular to the central axis of the insulating bushing. In some embodiments, the fasteners are inserted into the exterior surface of the insulating bushing mounting ring and are attached to the tank by threaded openings within the tank neck or nozzle. In other embodiments, the fasteners are inserted into threaded openings in the insulator bushing mounting ring and engage the tank. The fasteners are positioned equidistantly with respect to each other around the perimeter, or circumference, of the base of the insulating body or tube to uniformly attach the insulating body to the tank and provide a high strength joint.
The number of fasteners is dependent upon the size of the insulating bushing or diameter of the insulating bushing at its base. For electrical switching devices of the present invention, the number of fasteners is preferably between 4 and 32, or more preferably between 8 and 18, to provide reliable performance and properly distribute the applied load.
The fasteners used to join the insulating body and tank in the present invention can be standard hardware, such as, but not limited to, retention bolt and nut combinations, screws, screw and nut combinations, screw or bolt and washer combinations, or rivets. In one embodiment of the present invention, the insulating bushing and tank are joined together with conical-shaped washers and bolt combinations. The bolts are inserted substantially perpendicular to the surface of the insulating bushing and engage threaded openings within the surface of the tank.
Other, more preferred embodiments, use a combination of customized fasteners and minor modifications to the insulating bushing and tank, proximate to the point of joinder, to achieve a strong joint. In a preferred embodiment, the fasteners are a combination of customized bolts, that have conical-shaped heads on the bolt ends, and a locking nut. The conical shaped heads are inserted through a locking nut and then through threaded holes in the lower mounting ring of the insulating bushing. The conical end of the bolt then engages a machined groove in the tank nozzle that is within close proximity to the opening of the tank.
The present invention also discloses exterior housing, or weather covers, for current transformers that are mounted external to the insulating bushings with an improved design. The exterior housing of the present invention, which is preferably comprised of a metal such as spun aluminum, completely enclose the current transformers and minimize such problems as moisture pooling and environmental exposure. One embodiment of the present invention also allows for easy accessibility to the current transformers without complete disassembly of the exterior housing.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. The accompanying drawings are included to provide a further understanding of the invention. In the drawings, like reference characters denote similar elements throughout several views. It is to be understood that various elements of the drawings are not intended to be drawn to scale.